A. Field of the Invention
This invention relates to an improved method of forming semiconductor integrated circuit devices and more particularly to an improved method of forming silicided junctions as part of such devices.
B. Related Art
Silicides provide low sheet and contact resistance in deep sub-micron MOSFETs (MOSFETS having junction depths smaller than 1500 Angstroms). Self-aligned silicide processes are useful in deep sub-micron MOSFETS. The drive-in of dopants from silicides also lowers the generation of defect induced junction leakage current.
The silicide formation at the silicon surface is determined by the properties of the metal silicon interface. The deposition of highly pure metal and a clean surface are required for silicide reaction. Any residues or contaminants lead to non-uniform and rough silicide layers.
The process of integration requires the formation of a stable silicide phase. Metastable silicide layers may interfere with adjacent layers and influence the performance of the devices. The mechanism of the silicide formation and the thermal budget of the integrated process determine the grain size of the Cobalt silicides. A high surface energy of Cobalt silicide grains leads to an agglomeration of large silicide grains during the annealing process. The grains may even penetrate the diffusion layer. Eventually, an epitaxial regrowth of Silicon between the silicide grains degrades the silicide layers. Leakage currents of such silicided junctions are increased by some orders of magnitude.
Conventionally, the cleaning of the silicon surface is performed with hydro fluoric acid (HF) prior to the metal deposition. The thermodynamically stable CoSi2 phase is formed at an annealing temperature of 750 degrees Celsius (30 seconds, rapid thermal annealing RTA) via the Co2Si and CoSi phases. This results in a rough silicide/silicon interface. Further processing, such as ion implantation into the silicide, can lead to damage of the silicon crystal if the silicide film is pulled of the corner of the diffusion windows. The large grains agglomerate at higher annealing temperatures, i.e. during the drive in of the dopants.
Neither an HF dip, and HF bath or HF spraying procedure guarantee a uniformly clean silicon surface in diffusion windows (adhesion, surface energy). Cleaning the corners of diffusion windows efficiently is especially problematic. The disadvantages are rough silicide/silicon interfaces and highly stressed silicide layers which will often lead to heavy agglomeration, layer degradation and leaky junctions.